Naturally occurring gamma radiation for K-U-T (potassium, uranium and thorium) elements yields gamma ray intensity vs. energy spectra in the vicinity of a well borehole observed by highly stable scintillation detectors in the well borehole. The radiation at the scintillation detector and its associated photomultiplier produces a pulse height spectrum proportional to the energy of gamma rays impinging on the scintillation crystal. The spectrum is divided into selected energy ranges or windows. Three windows are centered about selected gamma ray emission peaks for the naturally occurring gamma rays of the K-U-T elements. Gamma ray count rates in each of the three energy ranges are transmitted to the surface and processed by a technique known as spectrum stripping wherein standard calibration spectra, for each of the individual elements (obtained in standard boreholes) are applied to the unprocessed data (count rates) of the selected windows (energy ranges) to detect each of the three elements of interest. The "stripping constants" are derived from measurements of the standard gamma ray energy spectra in standard boreholes containing essentially only one of the three elements to enable the stripping constants to be applied to the measured spectrum in an unknown earth formation surrounding a borehole. The concentration of the three elements of interest is determined after application of the stripping constants. After carrying out specified procedures, relative elemental concentrations of the K-U-T elements are obtained. Formation density may be derived from the borehole compensated count rates as well.
The actual gamma ray count rate achieved at a scintillation detector in a well borehole is dependent on the Compton attenuation coefficient .eta.. Each photon has a point of origination somewhere in the adjacent earth formations in traveling toward the scintillation detector. The attenuation of the gamma ray photon flux along the path of travel is dependent on the thickness of the material, the density of the material and the formation matrix type of the material. The gamma ray photons travel along a path having a length which is statistically determined from the distributed emission sources, namely the K-U-T elements. The present invention provides a measurement of formation density of an adjacent formation by utilization of the measured natural gamma ray spectrum observed at a scintillation crystal coupled with signal processing procedures as described below.
Major attenuation factors of the gamma ray flux include pair production, Compton scattering, and photoelectric absorption. Below certain energy levels, pair production is negligible and, therefore, not significantly involved in the method described herein.